Catalyst and process using the catalyst

ABSTRACT

A new chromium-containing fluorination catalyst is described. The catalyst comprises an amount of zinc that promotes activity. The zinc is contained in aggregates which have a size across their largest dimension of up to 1 micron. The aggregates are distributed throughout at least the surface region of the catalyst and greater than 40 weight % of the aggregates contain a concentration of zinc that is within ±1 weight % of the modal concentration of zinc in those aggregates.

RELATED APPLICATIONS

This application is a continuation of co-pending patent application Ser.No. 12/737,956 filed 18 Oct. 2011, which is a 371 national phase filingof PCT/GB2009/002124 filed 4 Sep. 2009.

BACKGROUND OF THE INVENTION

This invention relates to a chromium-containing fluorination catalystand to a process for the production of fluorinated hydrocarbons thatuses the catalyst. More particularly, the invention relates to a zincpromoted, chromium-containing fluorination catalyst and to a process forthe production of a fluorinated hydrocarbon in which a hydrocarbon orhalogenated hydrocarbon is reacted with hydrogen fluoride in thepresence of the catalyst.

The production of fluorinated hydrocarbons, which may also containhalogen atoms other than fluorine, by the catalysed vapour-phasefluorination of hydrocarbons or halogenated hydrocarbons with hydrogenfluoride is well known and numerous catalysts have been proposed for usein such processes. Catalysts containing and typically based on chromium,and in particular chromia, are frequently employed in the knownprocesses. Thus, for example, chromia or a halogenated chromia may beused in the vapour-phase reaction of trichloroethylene with hydrogenfluoride to produce 1-chloro-2,2,2-trifluoroethane as described inGB-1,307,224 and in the vapour-phase reaction of1-chloro-2,2,2-trifluoroethane with hydrogen fluoride to produce1,1,1,2-tetrafluoroethane as described in GB-1,589,924. The samecatalyst may also be used for the fluorination of chlorodifluoroethyleneto 1-chloro-2,2,2-trifluoroethane, for example in a process for theremoval of chlorodifluoroethylene impurity from1,1,1,2-tetrafluoroethane as also described in GB-1,589,924.

EP-A-0502605 discloses a chromium-containing fluorination catalyst whichcomprises an activity-promoting amount of zinc or a compound of zinc.The catalyst can be used in a process for preparing1,1,1,2-tetrafluoroethane in which 1-chloro-2,2,2-trifluoroethane isreacted with hydrogen fluoride in the presence of the catalyst toproduce the 1,1,1,2-tetrafluoroethane. The1-chloro-2,2,2-trifluoroethane may be prepared by reactingtrichloroethylene with hydrogen fluoride in the presence of the samecatalyst.

Manufacturers of fluorinated hydrocarbons are always seeking improvedcatalysts for use in the manufacture of those compounds. It has now beenfound that the stability of chromium-containing catalysts incorporatingcontrolled amounts of zinc may be improved if the distribution of zincin the catalyst meets certain criteria.

According to the present invention there is provided achromium-containing fluorination catalyst which comprises an amount ofzinc, said zinc being contained in aggregates which have a size acrosstheir largest dimension of up to 1 micron and which are distributedthroughout at least the surface region of the catalyst and whereingreater than 40 weight % of the aggregates contain a concentration ofzinc that is within ±1 weight % of the modal concentration of zinc inthose aggregates.

The present invention also provides a process for the production offluorinated hydrocarbons which comprises reacting a hydrocarbon or ahalogenated hydrocarbon with hydrogen fluoride in the vapour phase inthe presence of a fluorination catalyst as herein defined.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a three dimensional bar graphical illustration of zincdistribution across the surface of a typical sample of Catalyst A,identifying frequency of aggregate 0.5-1.0 micron in size and subvolumes20-30 nm in diameter relative to the percentage of ZnO.

FIG. 2 is a three dimensional bar graphical illustration of zincdistribution across the surface of a typical sample of Catalyst B,identifying frequency of aggregate 0.5-1.0 micron in size and subvolumes20-30 nm in diameter relative to the percentage of ZnO.

FIG. 3 is a linear graphical comparison of the stability betweenCatalysts A and B illustrated by the temperature required to achieve 10%133a to 134a conversion over a series of simulated operating cycles.

The zinc in the chromium-containing fluorination catalysts of thepresent invention is contained in aggregates that are evenly distributedthroughout at least the surface region of the catalyst. By the surfaceregion of the catalyst, we are intending to refer to that portion of thecatalyst that will contact the hydrogen fluoride and organic reactantsduring use. The surface of a catalyst is generally that region where thecoordination or valency of the atoms is not satisfied when compared tothe bulk material. Preferably, the zinc-containing aggregates are evenlydistributed throughout the entire catalyst bulk. The aggregates have asize across their largest dimension of up to 1 micron (1 μm), preferablyin the range of from 20 nm to 1 μm, and greater than 40 weight %,preferably greater than 50 weight %, more preferably greater than 60weight % and especially greater 70 weight % of the aggregates contain aconcentration of zinc that is within ±1 weight % of the modalconcentration of zinc in those aggregates. In a preferred embodiment,greater than 80 weight %, more preferably greater than 85 weight %, andespecially greater than 90 weight % of the aggregates contain aconcentration of zinc that is within ±2 weight % of the modalconcentration of zinc in those aggregates.

The modal concentration of zinc in the aggregates is that concentrationof zinc that occurs most frequently in the aggregates expressed as awhole number.

By ‘evenly distributed’ we include ‘substantially evenly distributed’and mean that the number or density of zinc-containing aggregates ineach region of the catalyst surface or the catalyst bulk, where the zincis dispersed throughout the entire catalyst, is substantially the same.For example, where the aggregates are only present at the catalystsurface, the number of aggregates in each square millimetre of thecatalyst surface is within ±2% of the mean number of aggregates persquare millimetre of the catalyst surface. Where the zinc-containingaggregates are distributed throughout the entire catalyst bulk, thenumber of aggregates in each square millimetre of the catalyst bulk iswithin ±2% of the mean number of aggregates per square millimetre of thecatalyst bulk.

In a preferred embodiment, the chromium-containing fluorinationcatalysts of the invention comprise one or more compounds selected fromthe chromium oxides, the chromium fluorides, fluorinated chromium oxidesand the chromium oxyfluorides.

The chromium compounds which make up the chromium-containing catalyst ofthe invention can contain chromium in any of its usual oxidation states,namely chromium (II), chromium (ill) and chromium (VI). However, thebulk or perhaps all of the chromium compounds in the catalysts willusually be based on chromium (III), although in a preferred embodimentfrom 0.1 to 8.0% by weight of the chromium based on the total weight ofchromium in the catalyst will be present as chromium (VI). Theperformance, particularly the activity and stability, of zinc promotedchromium-containing catalysts can be improved if some of the chromium inthe catalyst is present as chromium (VI)

Chromium (III) may typically comprise from 92.0 to 100% by weight,preferably from 94.0 to 100% by weight and particularly from 96.0 to100% by weight of the total weight of chromium in the catalyst. Wherechromium (VI) is also present, the catalyst of the invention typicallycomprises from 92.0 to 99.9% by weight, preferably from 94.0 to 99.9% byweight, e.g. from 95.0 to 99.5% by weight, particularly from 96.0 to99.5% by weight and especially from 96.0 to 99.0% by weight, e.g. from98.0 to 99.0% by weight of chromium (III) and from 0.1 to 8.0% byweight, preferably from 0.1 to 6.0% by weight, e.g. from 0.5 to 5.0% byweight, particularly from 0.5 to 4.0% by weight and especially from 1.0to 4.0% by weight, e.g. from 1.0 to 2.0% by weight of chromium (VI)based on the total weight of chromium in the catalyst. As all thechromium is usually present as chromium compounds, the percentagesquoted above will also normally define the amounts of chromium (III) andchromium (VI) compounds in the catalyst based on the total weight ofchromium compounds.

Chromium (III) compounds that may be present in the chromium-containingcatalyst of the invention include compounds selected from the groupconsisting of chromium (III) hydroxide, chromia (i.e. chromium (III)oxide), chromium (III) fluoride, fluorinated chromia and chromium (III)oxyfluorides. Chromium (VI) compounds that may be present in thecatalyst include compounds selected from the group consisting ofchromium (VI) oxide, chromic acid, fluorinated chromium (VI) oxide,chromium (VI) oxyfluorides and chromyl fluoride. The catalyst preferablycontains one or more chromium (III) compounds and one or more chromium(VI) compounds selected from the above groups of compounds. The preciseconstitution of the catalyst will depend, inter alia, on the methodsused for its preparation and whether the composition of the catalyst ismeasured pre- or post-fluorination.

Before the catalyst of the present invention is used in a fluorinationprocess or before it is subjected to a fluorination pre-treatment, asignificant proportion of the chromium, e.g. in excess of 50.0 weight %and more typically in excess of 75.0 weight % based on the total weightof chromium in the catalyst, is preferably present in the catalyst aschromium oxides, including chromia and preferably chromium (VI) oxide.It may also contain an amount of chromium hydroxides, including chromium(III) hydroxide and perhaps chromium (VI) hydroxide. The amounts of thechromium (III) oxides and hydroxides combined and, when present, theamounts of the chromium (VI) oxides and hydroxides combined arepreferably as discussed above for the chromium (III) and chromium (VI)compounds generally. A preferred chromium-containing catalyst,pre-fluorination, has a molar ratio of chromium (III) to oxygen tohydroxyl species (Cr (III):O:OH) in the range of from 1:0.5:2 to1:1.5:0, preferably in the range of from 1:1:1 to 1:1.5:0. This ratiocan be readily determined using thermogravimetric analysis. In oneparticular embodiment, the chromium-containing catalyst,pre-fluorination, has a molar ratio of chromium (III) to oxygen tohydroxyl species (Cr (III):O:OH) in the range of from 1:0.5:2 to 1:n:m,preferably in the range of from 1:1:1 to 1:n:m, where n is less than1.5, m is greater than zero and 2n+m=3.0.

When the catalyst is used in a fluorination process, or when it issubjected to a fluorination pre-treatment to be described hereinafter, aproportion of the chromium oxides in the catalyst and any chromiumhydroxides that may be present will be converted to chromium fluorides,fluorinated chromium oxides and/or chromium oxyfluorides.

The zinc/chromia catalysts used in the present invention may beamorphous. By this we mean that the catalyst does not demonstratesubstantial crystalline characteristics when analysed, for example, byX-ray diffraction.

Alternatively, the catalysts may be partially crystalline. By this wemean that from 0.1 to 50% by weight of the catalyst is in the form ofone or more crystalline compounds of chromium and/or one or morecrystalline compounds of zinc. If a partially crystalline catalyst isused, it preferably contains from 0.2 to 25% by weight, more preferablyfrom 0.3 to 10% by weight, and particularly from 0.4 to 5% by weight ofone or more crystalline compounds of chromium and/or one or morecrystalline compounds of zinc.

The amount of crystalline material in the catalysts of the invention canbe determined by any suitable method known in the art. Suitable methodsinclude X-ray diffraction (XRD). When XRD diffraction is used, theamount of crystalline material, such as the amount of crystallinechromium oxide, can be determined with reference to a known amount ofgraphite present in the catalyst (e.g. graphite used in producingcatalyst pellets) or, more preferably, by comparison of the intensity ofthe XRD patterns of the sample materials with reference materialsprepared by suitable internationally recognised bodies, for example NIST(National Institute of Standards and Technology), that contain a knownamount of a crystalline material.

The zinc is usually present in the catalyst as a zinc compound and maybe present in or on the chromium-containing catalyst, that is the zincor zinc compound may be incorporated in the chromium-containing catalystor it may be supported on the surface of the catalyst, depending atleast to some extent upon the particular method employed for preparingthe catalyst. If the zinc is incorporated throughout thechromium-containing catalyst, as is preferred, then it should besubstantially evenly distributed throughout the catalyst bulk.

The zinc is typically present in the catalyst in an amount of from 0.5to 25% by weight based on the total weight of the catalyst. The amountof zinc is important, because at the right levels it will promote theactivity of the chromium-containing catalyst. Too much zinc, on theother hand, may result in a decrease rather than an increase in catalystactivity.

The amount of zinc which will promote catalyst activity and produce acatalyst having an activity that is greater than the chromium-containingcatalyst alone depends, at least to some extent, on the surface area ofthe catalyst and whether the zinc is incorporated throughout thecatalyst bulk or just supported on its surface. Generally, the largerthe working surface area of the catalyst, the greater is the amount ofzinc which will be required to promote catalyst activity. Furthermore,catalysts containing zinc incorporated throughout their bulk, i.e. atsurface and non-surface locations, will tend to require larger amountsof zinc than those catalysts which only have zinc on their surface.

By way of example, in the case of a catalyst where the zinc isintroduced by impregnation to reside predominantly at the catalystsurface, activity promoting amounts of zinc for a chromium-containingcatalyst having a working surface area of between 20 and 50 m²/g areusually in the range of from about 0.5% to about 6.0% by weight based onthe total weight of the catalyst, preferably in the range of from about1.0% to about 5.0% by weight and especially in the range of from about2.0% to about 4.0% by weight.

However, for catalysts having larger working surface areas, for examplegreater than 100 m²/g, and comprising zinc distributed throughout thecatalyst bulk, the zinc may be present in an amount of from 5.0% to25.0% by weight based on the total weight of the catalyst, preferably inan amount of from 5.0 to 20.0% by weight and especially in an amount offrom 5.0 to 10.0% by weight.

For catalysts having small working surface areas, i.e. less than 20m²/g, for example about 5 m²/g, the amount of zinc may be as low as 0.5%to 1% by weight based on the total weight of the catalyst.

It should be understood that the amounts of zinc discussed above referto the amount of zinc itself, whether present as elemental zinc or as acompound of zinc. Thus, where the zinc is present as a compound of zinc,as is usual, the amounts refer to the zinc provided by the zinc compoundand not to the amount of the compound of zinc.

Preferred catalysts of the invention have a surface area in the range offrom 20.0 to 300.0 m²/g, more preferably in the range of from 100 to 250m²/g and particularly in the range of from 180 to 220 m²/g. Whenreferring to the surface area of the catalyst, we are referring to thesurface area prior to any fluorination treatment when measured by BETnitrogen isotherm (see, for example, G C Bond, HeterogeneousCatalysis—Principles and Applications 1987). These catalysts preferablycomprise from 0.5 to 25.0% by weight, more preferably from 0.5 to 10.0%by weight and particularly from 1.0 to 6.0% by weight of zinc based onthe total weight of the catalyst. The zinc can be distributed throughoutthe catalyst at surface and non-surface locations or just at thesurface.

Although the amount of zinc which will promote catalyst activity willvary depending, inter alia, on the surface area of the catalyst, uponthe distribution of zinc in the catalyst and upon the method that isused to prepare the catalyst, for any particular catalyst and catalystpreparation method, the amount of zinc that will promote catalystactivity is readily determined by routine experimentation using theabove percentages as a guide.

The chromium-containing catalyst may also comprise metal oxides,fluorinated metal oxides, metal fluorides or metal oxyfluorides otherthan chromium oxides, fluorinated chromium oxides, chromium fluorides orchromium oxyfluorides. The additional metal oxide may, for example, beselected from alumina, magnesia and zirconia, and in particular magnesiaand alumina, which during operation of the catalyst may be converted atleast in part to aluminium fluoride and magnesium fluoride respectively.

If desired, the catalyst may also contain one or more metals other thanzinc, for example nickel, cobalt or other divalent metals. Preferably,however, the chromium-containing catalyst will comprise just zinc,either as a metal but more typically as one or more zinc compounds.

The chromium-containing catalyst of the invention may also be supportedon a catalyst support material such as activated carbon or alumina.

The zinc promoter may be introduced into and/or onto thechromium-containing catalyst in the form of a compound, for example ahalide, oxyhalide, oxide or hydroxide, depending at least to some extentupon the catalyst preparation technique employed. When the zinc promoteris introduced by impregnating a chromium-containing catalyst, e.g. onecontaining one or more chromium (III) compounds and optionally one ormore chromium (VI) compounds, with a zinc compound, the zinc compound ispreferably a water-soluble salt, for example a halide, nitrate orcarbonate, and is impregnated into the chromium-containing catalyst bycontacting the catalyst with an aqueous solution or slurry of the zinccompound.

In an alternative method for preparing the catalyst of the invention,the hydroxides of zinc and chromium are co-precipitated and thenconverted to their oxides by calcination to prepare a mixed oxidecatalyst.

If other metal oxides are to be included in the catalyst, such asalumina, then these can be introduced by co-precipitating the hydroxidesof chromium and the other metal and then converting the hydroxides totheir oxides by calcination to prepare a mixed oxide catalyst, e.g. ofchromium and aluminium oxides such as chromia and alumina. Zinc can beintroduced into the catalyst by impregnating the hydroxide or oxidemixture with an aqueous solution or dispersion of a zinc compound in themanner discussed above. Alternatively, zinc hydroxide can beco-precipitated with the hydroxides of chromium and the other metal andthe three hydroxides then converted simultaneously to their oxides bycalcination.

In a preferred embodiment, the catalysts of the present invention areprepared by adding zinc and chromium (III) salts to water and thenco-precipitating the hydroxides of zinc and chromium (III) by adding asuitable inorganic hydroxide and preferably ammonium hydroxide to theaqueous salt solution. The co-precipitation is conducted under mixingconditions that will result in the zinc being evenly distributedthroughout the catalyst. The mixture of zinc and chromium hydroxides isthen collected, e.g. by filtration, washed and calcined to convert thehydroxides to their oxides. Any water soluble and stable salts of zincand chromium can be used including the chlorides, carbonates andnitrates. Preferred salts of chromium include chromium nitrate and basicchromium nitrate (Cr(NO₃)₂.OH). A particularly suitable chromium salt ischromium (III) nitrate. A preferred zinc salt is zinc nitrate.

The washing process following collection of the mixed hydroxideprecipitate can be important, because if the precipitate is preparedfrom a solution containing a nitrate salt then any nitrate that remainsfollowing the washing process can act as an oxidant to generate chromium(VI) from chromium (III) during the calcination process. The presence ofa small amount of chromium (VI) in the catalyst can further improvecatalyst activity and stability. More thorough washing of the collectedprecipitate, e.g. by repeated washing using fresh batches of washingliquor, will tend to reduce the residual nitrate levels and hence theamount of nitrate that is available to oxidise the chromium (III) duringthe calcination step. Furthermore, the nature of the washing medium caninfluence the efficacy with which nitrate contained in the mixedhydroxide precipitate is removed. For example, washing with an aqueousammonia solution is more effective at removing the nitrate than wateralone. Thus, if the mixed hydroxide precipitate is prepared from anaqueous solution containing chromium (III) and/or zinc nitrate, it ispossible to control the level of chromium (VI) in the catalyst followingcalcination by exercising control over the washing process, which inturn will affect the residual level of nitrate in the precipitate thatis available to oxidise the chromium (III).

Where a calcination step is employed in the production of the catalystsof the invention, as is preferred, it typically involves heating theprecursor catalyst material at a temperature in the range of from 300 to450° C., more preferably in the range of from 300 to 400° C., forexample around 350° C. The calcination may be conducted in an inertatmosphere, e.g. of nitrogen, or it may be conducted in air or in anatmosphere comprising air or oxygen in mixture with an inert gas such asnitrogen.

The calcination temperature that is used can also influence the level ofchromium (VI) in the final catalyst. For example, if the catalyst isprepared by calcining a mixed hydroxide precipitate prepared from anaqueous solution containing chromium (III) and/or zinc nitrate, then fora given level of residual nitrate following washing, higher calcinationtemperatures will tend to result in more of the chromium (III) beingoxidised to chromium (VI).

Another convenient way of generating a desired level of chromium (VI)compounds in the catalyst is by introducing a controlled amount of airinto the calcination step to oxidise the requisite proportion ofchromium (III) to chromium (VI). Here again, the calcination temperaturethat is used can also influence the level of chromium (VI) in the finalcatalyst, with higher calcination temperatures tending to encouragegreater oxidation of the chromium (III) for a given level of air.

The fluorination catalyst will usually be subjected to a fluorinationtreatment by heating in the presence of hydrogen fluoride, andoptionally an inert diluent, prior to being used in the catalysis offluorination reactions. A typical fluorination treatment comprisesheating the catalyst in the presence of hydrogen fluoride at atemperature in the range of from 250 to 450° C., more preferably in therange of from 300 to 380° C. and particularly in the range of from 350to 380° C. In a preferred embodiment, the fluorination treatment isconducted by contacting the fluorination catalyst with a mixture ofhydrogen fluoride and nitrogen. Conveniently, the treatment is conductedin the reactor in which the subsequent fluorination process is to beconducted by passing the hydrogen fluoride or hydrogen fluoride/nitrogenmixture through the reactor while it is heated.

Following the fluorination treatment, the working catalyst usuallycomprises at least a proportion of zinc fluoride in and/or on afluorinated chromium-containing catalyst material comprising one or morefluorine-containing chromium (III) compounds and preferably a smallamount of one or more fluorine-containing chromium (VI) compoundsselected from the fluorinated chromium oxides, the chromium fluoridesand the chromium oxyfluorides. Where the catalyst is a mixed oxidecatalyst prepared by co-precipitation of chromium and zinc hydroxidesfollowed by calcination to convert the hydroxides to their oxides, as ispreferred, the fluorination treatment usually converts at least aproportion of the oxides to oxyfluorides and fluorides.

The catalyst may be in the form of pellets or granules of appropriateshape and size for use in a fixed bed or a fluidised bed. Convenientlythe catalyst is in the form of cylindrically shaped pellets having alength and diameter in the range of from 1 to 6 mm, preferably in therange of from 2 to 4 mm, for example 3 mm.

After a period of use catalysing a fluorination reaction, the usedcatalyst may be regenerated or reactivated, for example by heating inair/nitrogen or air/hydrogen fluoride mixtures at a temperature of fromabout 300° C. to about 500° C. The regeneration or reactivation may beconducted periodically until the catalyst has reached the end of itsuseful lifetime. The catalyst may also be regenerated by passingchlorine through the reactor while heating the catalyst. Alternatively,the catalyst may be regenerated continuously while the process is beingoperated.

A further aspect of the present invention resides in the use of thezinc-promoted, chromium-containing catalyst in a fluorination process inwhich a hydrocarbon or halogenated hydrocarbon is reacted with hydrogenfluoride in the vapour-phase at elevated temperatures.

Accordingly, the present invention also provides a process for theproduction of fluorinated hydrocarbons which comprises reacting ahydrocarbon or a halogenated hydrocarbon with hydrogen fluoride atelevated temperature in the vapour phase in the presence of afluorination catalyst as herein defined.

Alkenes and alkanes as well as their halogenated counterparts containingat least one chlorine atom may be fluorinated using hydrogen fluorideand the catalysts of the present invention. Examples of specific vapourphase fluorinations which may be effected are the production of1,1,1,2-tetrafluoroethane from 1-chloro-2,2,2-trifluoroethane, theproduction of 1-chloro-2,2,2-trifluoroethane from trichloroethylene, theproduction of pentafluoroethane from dichlorotrifluoroethane, theproduction of dichlorotrifluoroethane, chlorotetrafluoroethane and/orpentafluoroethane from perchloroethylene and the conversion of1-chloro-2,2-difluoroethylene to 1-chloro-2,2,2-trifluoroethane.

The fluorination conditions employed when reacting the hydrocarbon orhalogenated hydrocarbon with hydrogen fluoride in the presence of thecatalyst of the invention may be those known in the art for fluorinationreactions that employ chromium-containing catalysts, for exampleatmospheric or super-atmospheric pressures and reactor temperatures inthe range of from 180° C. to about 500° C. When referring to the reactortemperature, we are referring to the mean temperature within thecatalyst bed. It will be appreciated that for an exothermic reaction,the inlet temperature will be lower than the mean temperature, and foran endothermic reaction, the inlet temperature will be greater than themean. The precise conditions will depend, of course, upon the particularfluorination reaction being carried out.

In a preferred embodiment, the catalyst of the invention is used in aprocess for preparing 1,1,1,2-tetrafluoroethane which comprises reacting1-chloro-2,2,2-trifluoroethane with hydrogen fluoride in the vapourphase at elevated temperatures in the presence of the catalyst. Reactiontemperatures in the range of from 250 to 500° C. are typically employed,with reaction temperatures in the range of from 280 to 400° C. beingpreferred and reaction temperatures in the range of from 300 to 350° C.being especially preferred. The process may be carried out underatmospheric or super-atmospheric pressures. Pressures of from 0 to 30barg are preferred whilst pressures of from 10 to 20 barg are especiallypreferred.

In a further preferred embodiment, the catalyst of the invention is usedin a multi-step process for preparing 1,1,1,2-tetrafluoroethane whichcomprises reacting trichloroethylene with hydrogen fluoride in thepresence of the catalyst to form 1-chloro-2,2,2-trifluoroethane. The1-chloro-2,2,2-trifluoroethane is then reacted with further hydrogenfluoride in the presence of the catalyst to form the1,1,1,2-tetrafluoroethane. The conversion of trichloroethylene to1-chloro-2,2,2-trifluoroethane and the conversion of1-chloro-2,2,2-trifluoroethane to 1,1,1,2-tetrafluoroethane may beconducted in discrete reaction zones of a single reactor, but they arepreferably conducted in different reactors. Both reactions are conductedat elevated temperatures in the vapour phase.

The preferred pressure and temperature conditions for the conversion of1-chloro-2,2,2-trifluoroethane to 1,1,1,2-tetrafluoroethane are asspecified above.

For the conversion of trichloroethylene to1-chloro-2,2,2-trifluoroethane, the process is typically conducted at atemperature in the range of from 180 to 300° C., preferably in the rangeof from 200 to 280° C. and particularly in the range of from 220 to 260°C. Atmospheric or super-atmospheric pressures may be employed in theprocess. Typically, the process is conducted at a pressure in the rangeof from 0 to 30 barg, preferably in the range of from 10 to 20 barg.

A particularly preferred embodiment of the above-described multi-stepprocess for preparing 1,1,1,2-tetrafluoroethane from trichloroethylenecomprises the steps of:

(A) in a first reaction zone reacting 1-chloro-2,2,2-trifluoroethanewith hydrogen fluoride in the vapour phase in the presence of afluorination catalyst of the invention at a temperature of from 250 to450° C. so as to form a product mixture containing1,1,1,2-tetrafluoroethane and hydrogen chloride together with unreactedstarting materials;

(B) conveying the total product mixture of step (A) as well astrichloroethylene and optionally further hydrogen fluoride to a secondreaction zone containing a fluorination catalyst of the invention and insaid second reaction zone reacting the trichloroethylene with hydrogenfluoride in the vapour phase at 180 to 350° C. to form1-chloro-2,2,2-trifluoroethane;

(C) collecting from step (B) a product mixture comprising1-chloro-2,2,2-trifluoroethane, 1,1,1,2-tetrafluoroethane and hydrogenchloride;

(D) treating the product of step (C) to recover1,1,1,2-tetrafluoroethane and produce a composition comprising1-chloro-2,2,2-trifluoroethane that is suitable for conveying to thefirst reaction zone in step (A);

(E) conveying the 1-chloro-2,2,2-trifluoroethane composition obtainedfrom step (D) optionally together with further hydrogen fluoride to saidfirst reaction zone; and

(F) collecting 1,1,1,2-tetrafluoroethane recovered in step (D).

Although the process described above refers to first and second reactionzones, this should not be taken as limiting the process to a particularorder. In chemical terms, trichloroethylene is first converted to1-chloro-2,2,2-trifluoroethane and the 1-chloro-2,2,2-trifluoroethane isthen subsequently converted to 1,1,1,2-tetrafluoroethane. Thus, thefirst reaction in the reaction sequence is the hydrofluorination oftrichloroethylene to form 1-chloro-2,2,2-trifluoroethane.

The first and second reaction zones may be provided by first and secondreactors or they may be discrete zones of a single reactor. Preferably,the first and second reaction zones are provided by first and secondreactors.

At least the stoichiometric amount of hydrogen fluoride is usuallyemployed in step (A) of the above process. Typically, from 1 to 10 molesof hydrogen fluoride and preferably from 1 to 6 moles of hydrogenfluoride are used per mole of 1-chloro-2,2,2-trifluoroethane.Accordingly, the product mixture of step (A) will usually containunreacted hydrogen fluoride in addition to 1,1,1,2-tetrafluoroethane,hydrogen chloride and by-products. It may also contain unreactedl-chloro-2,2,2-trifluoroethane. Preferred reaction temperatures for step(A) are in the range of from 250 to 500° C., more preferably in therange of from 280 to 400° C. and particularly in the range of from 300to 350° C. Preferred reaction pressures for step (A) are in the range offrom 0 to 30 barg, more preferably in the range of from 10 to 20 barg,for example around 15 barg. Preferred residence times in the firstreaction zone are in the range of from 1 to 600 seconds, more preferablyin the range of from 1 to 300 seconds and particularly in the range offrom 1 to 100 seconds.

In step (B), usually from 10 to 50 moles of hydrogen fluoride andpreferably from 12 to 30 moles of hydrogen fluoride per mole oftrichloroethylene are employed. Again, the reaction product of thisstage will normally contain unreacted hydrogen fluoride and may alsocontain unreacted trichloroethylene. Preferred reaction temperatures forstep (B) are in the range of from 180 to 300° C., more preferably in therange of from 200 to 300° C. and particularly in the range of from 220to 280° C. Preferred reaction pressures for step (B) are in the range offrom 0 to 30 barg, more preferably in the range of from 10 to 20 barg,for example around 15 barg. Preferred residence times in the firstreaction zone are in the range of from 1 to 600 seconds, more preferablyin the range of from 1 to 300 seconds and particularly in the range offrom 1 to 100 seconds.

Although the reactant mixtures that are conveyed to the first and secondreaction zones must include hydrogen fluoride, this does not mean that afresh or virgin supply of material has to be delivered to both reactionzones. For example, the process can be operated so that virgin hydrogenfluoride is only introduced into the second reaction zone in sufficientexcess that enough unreacted hydrogen fluoride can be recovered from theproduct mixture exiting step (B) to drive the hydrofluorination reactionthat occurs in the first reaction zone in step (A). One possibility isto operate step (D) of the process so that the1-chloro-2,2,2-trifluoroethane composition that is collected alsocontains hydrogen fluoride in a sufficient quantity for the reaction inthe first reaction zone. Alternatively, the process can be operated sothat virgin hydrogen fluoride is only introduced into the first reactionzone in sufficient excess that enough hydrogen fluoride remains in theproduct mixture of step (A) that is conveyed to the second reaction zonefor reaction with the trichloroethylene. Additionally, after start up,the hydrogen fluoride required for the hydrofluorination reactions inthe first and second reaction zones could even be introduced into adistillation column used to conduct step (D) of the process.

The reaction and separation steps which make up the preferred multi-stepprocess for making 1,1,1,2-tetrafluoroethane may be performed usingconventional equipment and techniques. Step (D), which is effectively aseparation/purification step in which the useable components making upthe product collected from step (B) are substantially separated from oneanother, may be effected by conventional distillation, phase separationand washing/scrubbing processes known to those skilled in the art.

The operation of the preferred multi-step process for making1,1,1,2-tetrafluoroethane is described more particularly inEP-A-0449617.

In another preferred embodiment, the catalyst of the invention is usedin a process for preparing pentafluoroethane which comprises reactingdichlorotrifluoroethane with hydrogen fluoride in the vapour phase atelevated temperatures in the presence of the catalyst. Reactiontemperatures of at least 280° C., e.g. in the range of from 280 to 400°C., are typically employed, with reaction temperatures in the range offrom 280 to 380° C. being preferred and reaction temperatures in therange of from 300 to 360° C. being especially preferred. The process maybe carried out under atmospheric or super-atmospheric pressures.Typically, the process is conducted at a pressure of from 0 to 30 barg,preferably at a pressure of from 12 to 22 barg and more preferably at apressure of from 14 to 20 barg.

In yet another preferred embodiment, the catalyst of the invention isused in a multi-step process for preparing pentafluoroethane whichcomprises reacting perchloroethylene with hydrogen fluoride in thepresence of the catalyst to form dichlorotrifluoroethane. Thedichlorotrifluoroethane is then reacted with further hydrogen fluoridein the presence of the catalyst to form the pentafluoroethane. Theconversion of perchloroethylene to dichlorotrifluoroethane and theconversion of dichlorotrifluoroethane to pentafluoroethane may beconducted in discrete reaction zones of a single reactor, but they arepreferably conducted in different reactors. Both reactions are conductedat elevated temperatures in the vapour phase.

The preferred pressure and temperature conditions for the conversion ofdichlorotrifluoroethane to pentafluoroethane are as specified above.

For the conversion of perchloroethylene to dichlorotrifluoroethane, theprocess is typically conducted at a temperature in the range of from 200to 350° C., preferably in the range of from 230 to 330° C. andparticularly in the range of from 240 to 310° C. Atmospheric orsuper-atmospheric pressures may be employed in the process. Typically,the process is conducted at a pressure in the range of from 0 to 30barg, preferably at a pressure in the range of from 10 to 20 barg andmore preferably at a pressure in the range of from 12 to 18 barg.

A particularly preferred embodiment of the above-described multi-stepprocess for preparing pentafluoroethane from perchloroethylene comprisesthe steps of:

(A) in a first reactor or a first plurality of reactors reactingperchloroethylene with hydrogen fluoride in the vapour phase at atemperature of from 200 to 350° C. in the presence of achromium-containing fluorination catalyst of the invention to produce acomposition comprising dichlorotrifluoroethane, hydrogen chloride,unreacted hydrogen fluoride and perchloroethylene, less than 2 weight %of chlorotetrafluoroethane and pentafluoroethane combined and less than5 weight % of compounds having the formula C₂Cl_(6-x)F_(x), where x isan integer of from 0 to 6, based on the total weight of organiccompounds in the composition;

(B) subjecting the composition from step (A) to a separation step toyield a purified composition comprising at least 95 weight % ofdichlorotrifluoroethane and less than 0.5 weight % of compounds havingthe formula C₂Cl_(6-x)F_(x), where x is an integer of from 0 to 6, basedon the total weight of organic compounds in the composition; and

(C) in a second reactor or a second plurality of reactors reacting thecomposition from step (B) with hydrogen fluoride in the vapour phase ata temperature of at least 280° C. in the presence of achromium-containing fluorination catalyst of the invention to produce acomposition comprising pentafluoroethane and less than 0.5 weight % ofchloropentafluoroethane, based on the total weight of organic compoundsin the composition.

By compounds of formula C₂Cl_(6-x)F_(x), where x is from 0 to 6, weinclude trichlorotrifluoroethane and dichlorotetrafluoroethane.

In step (A), from 3 to 50 moles of hydrogen fluoride are usuallyemployed per mole of perchloroethylene. Preferably, from 4 to 20 molesof hydrogen fluoride and more preferably from 4 to 10 moles of hydrogenfluoride are used per mole of perchloroethylene. Preferred reactiontemperatures and pressures for step (A) are as discussed above for theconversion of perchloroethylene to dichlorotrifluoroethane. Preferredresidence times for the reactants in the first reactor in step (A) arein the range of from 10 to 200 seconds, more preferably in the range offrom 30 to 150 seconds and particularly in the range of from 60 to 100seconds.

In step (C), from 2 to 20 moles of hydrogen fluoride are usuallyemployed per mole of dichlorotrifluoroethane.

Preferably, from 2 to 10 moles of hydrogen fluoride and more preferablyfrom 2 to 6 moles of hydrogen fluoride are used per mole ofdichlorotrifluoroethane. Preferred reaction temperatures and pressuresfor step (C) are as discussed above for the conversion ofdichlorotrifluoroethane to pentafluoroethane. Preferred residence timesfor the reactants in the second reactor in step (C) are in the range offrom 10 to 200 seconds, more preferably in the range of from 20 to 100seconds and particularly in the range of from 30 to 60 seconds.

The reaction and separation steps which make up the preferred multi-stepprocess for making pentafluoroethane may be performed using conventionalequipment and techniques. Separation step (B) may, for example, beeffected using conventional distillation, phase separation andwashing/scrubbing processes known to those skilled in the art.

The operation of the preferred multi-step process for makingpentafluoroethane is described more particularly in WO 2007/068962.

It is preferred to operate processes that use the catalyst of theinvention continuously, except for any shut-down time that is necessaryto regenerate or reactivate a catalyst that has been deactivated thoughuse. The feeding of air to the catalyst during operation of the processmay counter catalyst deactivation and reduce the frequency of processshut-downs for catalyst regeneration.

The present invention is now illustrated but not limited by thefollowing examples.

EXAMPLE 1 Catalyst Preparation

A catalyst sample (Catalyst A) was prepared on a 1 tonne/day scale asfollows.

A mixture of zinc and chromium (III) hydroxides was made byco-precipitation from an aqueous solution of zinc and chromium (III)nitrates using ammonium hydroxide (12.5% w/w ammonia in deionisedwater). The solution of zinc and chromium nitrates contained a chromiumcontent of approximately 10% w/w and a zinc content of approximately1.3% w/w to achieve a loading of zinc in the finished catalystformulation of around 8.0 weight %.

The equipment employed comprised a cooled and stirred precipitation tankwhich was fed with an aqueous stream comprising the zinc and chromiumnitrates and a separate stream of ammonium hydroxide. The tank stirrerwas rotated at 500 rpm during catalyst preparation. The mixed-nitratesfeed and ammonium hydroxide feed were injected continuously into thetank at a point close to the stirrer blade to ensure rapid mixing. Themixed-hydroxide product formed in the precipitation tank was collectedat an overflow point which maintained a constant slurry volume in theprecipitation tank during a catalyst preparation. The vessel walls werecooled to maintain a constant temperature and the ammonium hydroxidepumping rate adjusted to maintain the pH of the slurry in the range of 7to 8.5.

Slurry from the precipitation tank was filtered to recover theco-precipitated hydroxide mixture, which was then washed and filteredfurther.

Batches of washed solid were then dried at elevated temperaturesovernight in a nitrogen atmosphere, crushed to a powder, mixed with 2%w/w graphite and compacted to form pellets of about 6 mm in diameter andlength. The compacted pellets were then calcined at 300° C. under a flowof nitrogen.

Measurement of Zinc Distribution

The catalyst produced was sampled and the zinc distribution across thesurface of the catalyst samples measured by Energy Dispersive Analysisof X-rays (EDAX) using a Transmission Electron Microscope.

The zinc distribution for a typical sample of Catalyst A is shown inFIG. 1.

Catalyst Testing:

Catalyst A was used to catalyse the reaction of1-chloro-2,2,2-trifluoroethane with HF to produce1,1,1,2-tetrafluoroethane and its stability investigated by measuringthe decrease in catalytic activity with use. The catalyst test rigcomprised a single Inconel reactor tube.

6 g of catalyst pellets reduced to a particle size of from 2.0 to 3.35mm was loaded into the Inconel reactor tube and dried at 250° C. and 3barg pressure under flowing nitrogen (80 ml/min) for 16 hours. Thecatalyst was then fluorinated using a mixture of nitrogen (80 ml/min)and HF (4 ml/min) at around 3 barg pressure. The fluorination processwas commenced at 300° C. and the reactor temperature was then ramped to380° C. at 25° C./hr. The temperature was maintained at 380° C. for afurther 7 hours.

After the 7 hours had elapsed, the nitrogen flow was stopped and thereactor was cooled to 315° C. with the HF flow still running.1-chloro-2,2,2-trifluoroethane (30 ml/min) and HF (90 ml/min) were thenfed to the reactor at a pressure of 14 barg. The temperature required toachieve a 10% conversion of 1-chloro-2,2,2-trifluoroethane to1,1,1,2-tetrafluoroethane was determined and recorded. The temperatureat which the 10% conversion occurred was taken to be a measure ofcatalyst activity—a more active catalyst allows the 10% conversion to beachieved at a lower reaction temperature.

After 48 hours of operation, the feed of 1-chloro-2,2,2-trifluoroethanewas stopped and the catalyst subjected to a regeneration cycle whichinvolved passing a mixture of HF (90 ml/min) and air (6 ml/min) throughthe reactor at a pressure of 14 barg while maintaining the reactor at atemperature of 380° C. This process was continued for at least 40 hours.

After completing the regeneration process, the flow of air was stoppedand the reactor cooled to 315° C. with the HF flow still running. Theflows of HF and 1-chloro-2,2,2-trifluoroethane were then recommenced andthe activity of the catalyst investigated in exactly the same manner asbefore. After the end of another 48 hour operating cycle, the catalystwas regenerated, once again, in the same manner as before.

The operating and regeneration cycles were repeated several times inorder to determine how the activity of the catalyst decreases with useand so obtain a measure of catalyst stability. Over the course of manyoperating and regeneration cycles, the catalyst loses activity and thetemperature required to achieve a 10% conversion of1-chloro-2,2,2-trifluoroethane to 1,1,1,2-tetrafluoroethane graduallyincreases. The increase in reactor temperature that is needed tocontinue to achieve the 10% conversion is a measure of catalyststability, as more stable catalysts are able to resist the cyclicthermochemical stressing better, resulting in smaller increases in thetemperature that is needed to achieve the 10% conversion.

The performance of Catalyst A is shown in FIG. 3.

EXAMPLE 2 Catalyst Preparation

A catalyst sample (Catalyst B) was prepared on a 1 tonne scale using thesame procedure as described above for preparing Catalyst A except that astatic mixer was placed in the chromium nitrate/zinc nitrate feed lineand the combined feed was ejected into the precipitation tank using ajet mixer in order to improve the zinc distribution in the finalcatalyst.

Measurement of Zinc Distribution

The catalyst produced was sampled and the zinc distribution across thesurface of the catalyst determined as before. The zinc distribution fora typical sample of Catalyst B is shown in FIG. 2.

Catalyst Testing:

Catalyst B was used to catalyse the reaction of1-chloro-2,2,2-trifluoroethane with HF and its stability investigated asdescribed above for Catalyst A.

The performance of Catalyst B is shown in FIG. 3. It is evident thatCatalyst B, with its improved zinc distribution, was more stable andbetter able to withstand the cyclic thermochemical stressing.

1. A process for preparing a chromium-containing fluorination catalystcomprising zinc, said method comprising the steps of: providing a tank;introducing an aqueous solution comprising a mixture of zinc andchromium nitrates into the tank using a jet mixer; introducing anaqueous hydroxide compound into the tank; collecting a mixed-hydroxideproduct comprising zinc hydroxide and chromium (III) hydroxide from thetank; drying the mixed hydroxide product collected from the tank;crushing the dried product to produce a powder; calcining themixed-hydroxide product to convert the zinc hydroxide and chromium (III)hydroxide to their oxides; and mixing the powder with graphite andcompacting the mixture to form pellets.
 2. The process of claim 1,wherein the tank comprises a rotating stirrer blade.
 3. The process ofclaim 2, wherein the aqueous solution comprising the mixture of zinc andchromium nitrates is introduced into the tank close to the stirrerblade.
 4. The process of claim 2, wherein the aqueous hydroxide compoundis introduced into the tank close to the stirrer blade.
 5. The processof claim 1, wherein the mixed hydroxide product collected from the tankis filtered and washed prior to being dried.
 6. The process of claim 1,wherein the aqueous hydroxide compound introduced into the tank isammonium hydroxide.
 7. The process of claim 6, wherein the rate at whichthe ammonium hydroxide is introduced into the tank is adjusted tomaintain a pH in the tank of from 7 to 8.5.
 8. The process of claim 1,further comprising the step of fluorinating the pellets.
 9. The processof claim 8, wherein the pellets are fluorinated using HF.
 10. Theprocess of claim 8, wherein the catalyst that is produced comprises oneor more chromium (III) compounds selected from the group consisting ofchromia, chromium (III) fluoride, fluorinated chromia and chromium (III)oxyfluorides.
 11. The process of claim 10, which produces a catalystcomprising from 0.5 to 25% by weight of zinc based on the total weightof the catalyst, said zinc being contained in aggregates which have asize across their largest dimension of up to 1 micron and which aredistributed throughout at least the surface region of the catalyst andwherein greater than 40 weight % of the zinc-containing aggregatescontain a concentration of zinc that is within ±1 weight % of the modalconcentration of zinc in the zinc-containing aggregates.
 12. The processof claim 11, which produces a catalyst in which greater than 60 weight %of the zinc-containing aggregates contain a concentration of zinc thatis within ±1 weight % of the modal concentration of zinc in thezinc-containing aggregates.
 13. The process of claim 11, which producesa catalyst in which greater than 80 weight % of the zinc-containingaggregates contain a concentration of zinc that is within ±2 weight % ofthe modal concentration of zinc in the zinc-containing aggregates. 14.The process of claim 11, which produces a catalyst in which greater than90 weight % of the zinc-containing aggregates contain a concentration ofzinc that is within ±2 weight % of the modal concentration of zinc inthe zinc-containing aggregates.
 15. The process of claim 11, whichproduces a catalyst in which the number of zinc-containing aggregates ineach square millimetre of the catalyst surface or in each squaremillimetre of the catalyst bulk is within ±2% of the mean number ofzinc-containing aggregates per square millimetre of the catalyst surfaceor catalyst bulk.
 16. The process of claim 11, which produces a catalystin which the zinc-containing aggregates have a size in the range of from20 nm to 1 micron.
 17. The process of claim 11, which produces acatalyst in which from 92.0 to 100% by weight of the chromium in thecatalyst based on the total weight of said chromium is present aschromium (III).
 18. The process of claim 11, wherein the catalyst thatis produced comprises one or more chromium (VI) compounds and whereinfrom 0.5 to 4.0% by weight of the chromium in the catalyst based on thetotal weight of said chromium is present as chromium (VI).
 19. Theprocess of claim 18, which produces a catalyst in which from 96.0 to99.5% by weight of the chromium in the catalyst based on the totalweight of said chromium is present as chromium (III).
 20. The process ofclaim 11, wherein the catalyst that is produced comprises zinc in anamount of from 1.0 to 6.0% by weight based on the total weight of thecatalyst.
 21. The process of claim 11, wherein the catalyst that isproduced is amorphous or partially crystalline containing from 0.4 to 5%by weight of one or more crystalline compounds of chromium and/or one ormore crystalline compounds of zinc.
 22. The process of claim 11, whereinthe catalyst that is produced has a surface area in the range of from 20to 300 m²/g.
 23. The process of claim 22, wherein the catalyst that isproduced has a surface area in the range of from 100 to 250 m²/g.